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F15Rules

Can we have a discussion on Diagonals??

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I'd be interested to hear other members' views on the plethora of different diagonals that are available..

I use both 2" (Meade 5000 enhanced dielectric) and 1.25" (various brands, Celestron, Pentax, Generic, Tal).

When you think of it, these items are crucial for most refractor and sct users...without them we get cricked necks and lie on our backs a lot:p!

I want to suggest that, unlike with Eyepieces, whereby as a general rule, you get what you pay for, it is different with star diagonals..I have used a number of different types, from quite expensive to very cheap, and in my honest opinion, I think that the benefits of the expensive ones versus the mainstream or even some cheaper ones are very dubious indeed.

As an example, I bought a Williams Optics 1.25" dielectric last year, in absolute mint conditon - it was like new, with posh box, paperwork etc. It was a beauty to look at, and had lovely grey carbon fibre sides to it. It looked like a high quality item..

I intended it to replace my trusty old Tal 1.25" diagonal, which had come with one of my Tal 4" refractors..I knew it was decent quality, but assumed the Williams Optics one, at 3 times the price new, and looking as it did, would blow the Tal away. But it didn't. It didn't look any better at all to my eyes...no worse, but no better either, despite the "dielectric" badge with "enhanced 99% reflectivity"..believe me, I WANTED the WO diagonal to be better, at the price. But it wasn't, so I sold it on (got back what I paid for it), and kept the Tal. And I had a new found respect for my old Tal, even though it could never be described as "pretty" like the WO...more like "industrial":p..

I've had similar experiences with other diagonals, and I'd be interested to hear others' views..but for me, I won't pay top dollar for posh looking diagonals in future, I will buy decent mid range ones, (often unbranded, but clearly coming out of the same Far Eastern factory!), and save any extra cash for better eyepieces, where the cost differences, to my eyes at least, can be seen visually through the scope!

cheers

Dave

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Nice discussion, F15.

I have to say, you're not the first to note this phenomenon! I have never directly compared two diagonals, but I know of several tests between diagonals of vastly differing "quality" that have found no differences.

I think thethings that make a good diagonal are solid build, a good system to hold the eyepiece securely and properly (i.e. tight fit, multiple set screws, or compression ring), and easy means to clean the mirror. All else is a luxury.

Saying that, I also bought a WO 1.25" diagonal to use with a particular eyepiece that needs the shorter light path of a 1.25", but is very large and heavy, so benefits from the excellent build of the WO. A cheapo one just wouldn't be able to handle the weight of the thing. I also intend to use my excellent Zeiss eyepieces in it, as I wouldn't want to find myself in a position where the diagonal is the limiting factor! So for security, I went posh and got the best i could afford...

Andrew

Edited by Andrew*

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I've never had the opportunity to try an expensive diagonal, but I do think that comparing the ones I have used, a real dirt cheapy all made of plastic (excepting the mirror) and a reasonable one with metal parts, I couldn't tell the difference in the view. Now... putting a Hyperion on the plastic one was a scary prospect... so I'd say it's worth paying that bit more to get a decent build quality.

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Any optical system is no better than its weakest component. There's no point in paying thousands of pounds for a premium instrument & skimping on the diagonal ... skimping on eyepieces for rarely used magnifications is a much more sensible proposition. Yet some people seem to be happy to spend £400 a pop on super premium EPs but resent paying a few quid extra on the diagonal....

Diagonals also need to be collimated. Poor collimation will spoil performance. Some make the task easy, some don't. Some have compression rings which don't match well with the cutouts in the barrel of your favourite eyepieces. Some even have no compression ring, nor even a nylon insert in the eyepice retaining screw tip.

You pays your money & takes your choice. I've found my WO diagonal to be pretty good but they do seem to be a bit variable. TV Everbrites are excellent when used with TV eyepieces, but TV seem to differ from most everyone else about compression ring / cutout placement & dimensions. For this reason I don't reccomend any particular diagonal.

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I have compared a cheap Meade prism, a TAL diagonal and a WO dilelectric diagonal.

The prism beats both of the diagonals and the Tal matches the much more expensive WO in terms of contrast, detail and light scatter.

Now Barlows are another story ......

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I totally agree with you Dave (et al).

I can see no difference whatsoever with my 2" WO dielectric when compared with the standard Celestron issue that came with the scope.

BUT as you hint at, I also bought the WO for the following reasons and in approximate order of importance:

  • Better image quality - didn't get that - although as you say it's no worse.
  • Excellent build quality and durability of mirror - got that - it will last me forever.
  • Fitted 2" eyepieces - my previous one was 1.25" - obviously got that!
  • Compression fittings (in both the 2" fitting and the 1.25" adapter) rather than a single thumbscrew - got that too.
  • The taper fit nosepiece, rather than undercut/straight side - also got that.
  • Lovely looks - definitely got that - it's nice!

Like you though, hand on heart, as long as the diagonal has compression fittings and is 2" (if your scope can accommodate it) I'd say that it really makes no difference which one you get.

Cheers

Shane

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I too agree with Dave. Unless you have tried them all you can't quote chapter and verse but I would say that I have never seen less return for your money than with costly diagonals. However, Brian points out the exasperating compression ring compatibility business and that does matter. Personally I wish they had never been invented.

Olly

Edited by ollypenrice

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Any optical system is no better than its weakest component. There's no point in paying thousands of pounds for a premium instrument & skimping on the diagonal ... skimping on eyepieces for rarely used magnifications is a much more sensible proposition. Yet some people seem to be happy to spend £400 a pop on super premium EPs but resent paying a few quid extra on the diagonal....

I don't think that you will get the full benefit of a high end diagonal unless the rest of your scope is up to the same standard. The best diagonal in the world won't improve the performance of your scope if it's the quality of it's optics that's the limiting factor. Also the high end diagonals do have better build quality, not just simply in fit and finish but in collimation.

I've been comparing an Astro Physics diagonal, WO dielectric and a Revelation Quartz and found it didn't make any real difference on a scope with just good optics. When used with my FLT98 and an Ethos eyepiece however the WO was slightly better than the Revelation (but there wasn't much in it). The Astro Physics was a little bit sharper and showed more subtle detail than the WO. And when you're trying to find that little bit more planetary or lunar detail or if you want to split double stars the difference is noticeable. The best way I can quantify the difference is that it''s similar to the improvement you get when going from a Nagler to an Ethos eyepiece. I mean the Nagler is a superb eyepiece but the Ethos shows you that little bit more.

John

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when I bought my celestron ED80 my first impression was that it was a dog because I couldn't split any close doubles with it and the diffraction pattern had strange vertical lines running through it. I changed the supplied diagonal for a celestron 2" jobbie (about 70 quid IIRC) and the scope started to perform as I had expected it to. So my experience (of a huge sample of two...) was that you get what you pay for.

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I've tried a few diagonals of various qualities over the years from Tele Vue Everbright's down to the cheapest of the cheap.

At the moment I use a Tele Vue Everbright 2" in one refractor and a 15 year old Meade (Japanese) 2" standard mirror diagonal in the other and have swapped them over a few times. The build quality of the Tele Vue is better - it's all machined from a single alloy block and has brass compression rings on both the 2" and 1.25" fittings. Optically I really can't tell the difference between them at all. The Tele Vue cost me 5x as much as the Meade did though :)

Incidentally I had a William Optics 1.25" dielectric diagonal for a while which was very nice until I noticed that the ring that attaches the eyepiece drawtube to the body of the diagonal was so thick that it vignetted the views with my Nagler eyepieces - so that had to go.

Edited by John

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I was able to try out quite a few 1.25" diagonals ranging from £40 to £170 on a three grand APO, and I could not see any difference

in the eyepiece with my eyes. The expensive one had better build quality, but then again the one costing £65 was also nice.

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This is my recent experience with diagonals:-

With the SW 66 Equinox I have used 3 diagonals. A Stellarvue 1.25 Dielectric, Agena uncoated prism diagonal and an Orion USA prism diagonal (coated). I was using the Stellavue Diagonal which gave good views. Then I read on another forum how prism diagonals improve contrast and colour correction with short tube telescopes with Fpl51 glass. I bought a very cheap prism and gave it a go and was suprised with the results. Viewing a very low Jupiter with the Stellarvue, I could see red fringing. When I swapped over to the prism, the fringing had disappeared and could see more detail on the belts of jupiter. Later I bought the Orion USA diagonal, this has coating on the prism and when compared prisms during the day, the Orion showed a slightly brighter and sharper image. I now only use the Orion diagonal with this telescope.

WO 80 FD: Again three diagonals have been tried with this telescope, though during the day only. These are the Stellarvue Dielectric, Agena Prism and the WO 2inch Quartz Dielectric.I tried the Stellarvue and Agena diagonals and could see no difference between them. When I tried the WO Quartz, I was surprised by the difference, I'll admit I was hoping for a subtle difference with the WO, but the difference was like someone blew all the dust and cobwebs away, as the image was alot brighter and sharper.

Bresser 70mm F10: Two diagonals for this one. the stock diagonal and the Agena prism diagonal. The stock diagonal was a very cheap plastic affair. The chrome barrel was plastic, the locking srew was plastic as was the mirror. This gave everthing a deep yellow cast. It was so cheap and nasty, I'd binned it. The Agena prism was a massive improvement. The colour was neutral and the view was brighter, clearer and sharper.

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I have no practical experience of different diagonals, but I've had a few thoughts on the subject from a theoretical point of view. There may be obvious mechanical housing differences and advantages between cheap and expensive diagonals so I'll ignore them for now and concentrate on the potential optical effects.

Some optical properties of a mirror are Flatness, Coating type, surface quality and substrate material. The flatness affects the aberrations in the system and the ultimate image quality. The coating type affects the spectral reflectance and its behaviour with angle of incidence. The quality of the coating and substrate surface will affect scattering and the durability of the coating. The substrate material itself will affect how well it can take a flat surface and maintain its qualities under environmental variations.

An expensive diagonal is likely to have the following advantages. The mirror will be flatter (maybe from a few wavelengths to a fraction of a wavelength) and any deviations will be symmetric rather than asymmetric. This will improve the resolution of the system and aid things like star splitting and planetary detail.

The coating will be of higher spectral reflectance, more uniform with wavelength and be able to maintain these qualities with high angles of incidence. This will improve the overall transmission of the system, improve colour balance and help maintain these qualities with low F/Number systems (high incidence angles). This will help the viewing of very dim targets, and will be less likely to act as a filter for spectral measurements.

The durability of the coating will be improved so it is less likely to absorb moisture and will withstand the extreme temperature cycling that can occur, perhaps between freezing and solar observing. The improved quality of the coating is less likely to have surface scattering defects on it, so there will be less scattering and greater contrast.

The substrate material will be improved, perhaps from float glass to low expansion borosilicate or zerodur so that there will be less expansion or distortion of the mirror. This will help the optical qualities noted and will reduce the temperature cyclic expansion and contraction of the surface and hence improve coating adhesion.

So there are potential advantages to be had if the manufacturer implements some of the quality enhancements described in higher priced diagonals. That said, the enhancements will probably only be apparent in high quality imaging systems that have been tuned to give the absolute best image quality, systems with a fast F/Number, or over the passing of time where the durability gives longer life.

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Does a WO diagonals optical quality match its looks? Too much feedback on the web indicates that WO produces have significant variability in image quality - but they do look good.

I had a very top of the range Dielectric diagonal, used with a totally colour free apo (TMB115) but then I tried the Baader Zeiss prism diagonal which was definitely better, like a veil had been lifted from the view, I think this is down to less light scatter. This prism induces no colour at all in scopes that are slower than F7, with faster scopes it possibly will.

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I have one and it's really nice but I cannot say that it really improved the view greatly from the cheapo one that came with my refractor.

as I say above though (post #6), image quality was only one factor in the decision for me.

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I only have two diagonals and my experience of refractors is quite limited. The two diagonals I have are the TAL 1.25" one which is built like a tank, doesnt look pretty but it feels as if you club someone to death with it and it would be none the worse for wear :) The other diagonal is a Revelation(GSO) 2".

I have tried thyem both in the TAL 100 with a range of EPs running from cheap and nasty through to Hyperions and I really cant say the views are any different between the two. That might be because the TAL is brilliant or that the Revelation is duff. Personally I suspect its the former and that the TAL diagonal punches well above its weight.

It might also be that the TAL 100 isnt a sufficently sohpisticated scope to see any difference no matter what you use. I cant say.

Edited by Astro_Baby

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The thing that I find interesting about this thread is that while some see significant differences when using high quality diagonals, some can't seem to tell the difference. Is it differences in peoples visual acuity that makes the difference or is it down to differences in equipment used or a bit of both?

John

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Is it differences in peoples visual acuity that makes the difference or is it down to differences in equipment used or a bit of both?

Both, I'd suspect - plus variations in the object viewed / imaged; nebulae etc. are pretty tolerant but planets are a much tougher test. I'd suspect a lot also depends on the state of cleanliness of the diagonal, whether the coatings have degraded and manufacturing / quality control standards.

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Both, I'd suspect - plus variations in the object viewed / imaged; nebulae etc. are pretty tolerant but planets are a much tougher test. I'd suspect a lot also depends on the state of cleanliness of the diagonal, whether the coatings have degraded and manufacturing / quality control standards.

all true I reckon - in my case probably also a lack of experience

I am a firm believer that the longer you look (both collectively and during each session) the more detail you 'learn' to see. unfortunately I sold my cheapo diagonal so cannot make a direct comparison.

I'm almost tempted to buy another from astroboot and test this out

:)

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Hi.I need to know one thing and its important.The diagonal wich comes with the celestron SCT like the nexstar,are a prism?Are the original celestron SCT diagonal prism?

Cheers

carlos

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Are the original celestron SCT diagonal prism?

Mine was.

First thing I did when I got my CPC1100 was replace the supplied 1.25" visual back & diagonal with 2" varieties. Not because the supplied diagonal is bad (it's actually quite good; I'm using it with my Solarscope) but because an 11" SCT needs a wider field of view at low power than is obtainable with a 1.25" back end.

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I had a very top of the range Dielectric diagonal, used with a totally colour free apo (TMB115) but then I tried the Baader Zeiss prism diagonal which was definitely better, like a veil had been lifted from the view, I think this is down to less light scatter. This prism induces no colour at all in scopes that are slower than F7, with faster scopes it possibly will.

True and not true. The cheaper prisms with Bak7 glass has been noted to improve colour correction with scopes using fpl51 glass at F5.9. I have read that Takahashi recomend do not use a prism at all with their TOA scopes as they are optimised for imaging and bringing in an extra glass element can cause CA. I suppose it a case of mating the right prism to the right lenses.

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Hi.Want to know a thing and its important for mi.Some say the prism is superior than the mirror diagonal for fracs at about F6.I .Would know how it compares prism vs mirror in a celestron 8" SCT since the original diagonal witch comes with the C8 is a prism.

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Would know how it compares prism vs mirror in a celestron 8" SCT

A good prism is preferable to a bad mirror & vice versa. Doesn't make a lot of difference.

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Thanks for all your contributions, some interesting stuff!

I'd offer the following having read them all..

-the construction of some diagonals is clearly much better than others, both actually and aesthetically..so if you want a diagonal that will still be very useable in 10-20 years, it might be worth investing the extra cash.

-however, many of us change our equipment quite regularly, including accessories. If you are one of those who do, is it worth investing a lot of money in high end accessories that you may not keep (on the basis that depreciation tends to hit more expensive purchases harder)?

-the point about differing visual acuity is interesting, but I don't know how you would really measure it..since we only use our own equipment for most of the time, I guess our own comparisons of one diagonal with another are relevant at least for ourselves, since our good or bad sight will affect both equally?

-try a comparison of viewing with no diagonal at all, ie straight through viewing a la Galileo..most early observations had to be done this way. Do you notice an improvement when the diagonal is removed altogether? Or do you find, as I suspect is the case for myself, that any potential improvement using straight through viewing is lost due to increased discomfort (ie cricked neck!) meaning that we don't want to linger on any one subject for long?

Thanks again to all who have read and contributed to this thread..

regards

Dave

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